1. Field of the Invention
The present invention relates to an ozone flow rate control device and, particularly, to an ozone flow rate control device for controlling flow rate of ozone gas containing nitrogen gas.
2. Description of the Related Art
The so-called atmospheric pressure CVD is one of methods for forming a thin film on a semiconductor wafer of a semiconductor device. For example, in the atmospheric pressure CVD, a thin film is formed by introducing ozone gas (O.sub.3) and a gas such as TEOS (Si(OC.sub.2 H.sub.5).sub.4) into a reaction chamber and depositing a reaction product of ozone gas and TEOS on a wafer. In order to obtain such thin film containing a required constituents, it is necessary to maintain a gas flow rate to be introduced into the reaction chamber at a predetermined value by a flow rate control device provided in a gas supply system of the atmospheric pressure CVD device. FIG. 1 is a block diagram showing an example of the gas supply system of the conventional atmospheric pressure CVD. The conventional technology will be described with reference to FIG. 1. In FIG. 1, air valves 2 and 3 for supplying oxygen gas (O.sub.2) and nitrogen gas (N.sub.2) to an ozone generator 1 are provided, respectively, and a mixture of oxygen gas and nitrogen gas is supplied to a mass flow meter (MFM) 12 for monitoring gas flow rate. In the nitrogen gas line, a mass flow controller (MFC) 4 for controlling nitrogen gas flow rate is connected to an input side of the air valve 3. An air valve 5 is connected to an output side of the ozone generator 1 and a mass flow controller 7 is connected to an input side of an ozone generator 7, for flowing ozone gas to an atmospheric pressure chemical vapor deposition (CVD) chamber 8. An air valve 6 which is alternately on-off controlled with respect to the air valve 5 is connected between the air valve 5 and the mass flow controller 7, for flowing nitrogen gas to the atmospheric pressure CVD chamber 8.
On the other hand, the TEOS gas supply system includes an air valve 9 and a mass flow controller 11 connected to the atmospheric pressure CVD chamber 8. An air valve 10 which is alternately on-off controlled with respect to the air valve 9 is connected between the air valve 9 and the mass flow controller 11 for supplying nitrogen gas to the atmospheric pressure CVD chamber 8.
The reason for the addition of nitrogen gas to oxygen gas in generating ozone will be described. It is possible to generate ozone from only oxygen gas. In such case, however, there is a reduction of ozone concentration with time and it is impossible to maintain ozone concentration high enough. On the other hand, it has been known that it is possible to make concentration of ozone gas higher than that generated from only oxygen gas without reduction of ozone concentration with time, by generating ozone gas by adding 1 to 10 volume % of nitrogen gas, helium gas, argon gas or carbon dioxide gas to oxygen gas. Generally, the gas to be added is nitrogen gas in view of the easiness to supply of nitrogen gas as a line gas of factories, low cost and possibility of maintaining high ozone concentration compared with other additive gases. Particularly, in growing a SiO.sub.2 film by using TEOS and ozone gas, ozone concentration must be maintained at about 140 g/Nm.sup.3. Therefore, the use of nitrogen gas as the additive gas is advantageous.
Now, an operation of the gas supply system of the atmospheric pressure CVD device will be described. Usually, in forming the thin film, incomplete reaction products are prevented from being formed when gas is introduced to the atmospheric pressure CVD chamber by supplying ozone gas to the atmospheric pressure CVD chamber and, after a constant time lapse while continuing the ozone gas supply, introducing TEOS gas to the chamber. First, nitrogen gas is introduced into the atmospheric pressure CVD chamber 8 under flow rate control of the mass flow controllers 7 and 11 by closing the air valves 2, 3, 5 and 9 with the ozone generator 1 being in an inoperable state and opening the air valves 6 and 10. When the thin film is formed, the air valve 6 is closed and the air valves 2, 3 and 5 are opened to supply, together with oxygen gas, nitrogen gas to the ozone generator 1 at a flow rate determined by the mass flow controller 4 to, for example, 1 volume % of the flow rate of oxygen gas which is determined by the mass flow controllers 7 and 4. In this state, the ozone generator 1 is activated, so that the oxygen gas is converted into ozone having a predetermined concentration and introduced into the atmospheric pressure CVD chamber 8. In this case, it can be judged whether or not the mass flow controller 7 is normally performing the flow rate control, by displaying and comparing the flow rates of the mass flow meter 12 and the mass flow controller 7. The air valves 10 and 9 are closed and opened, respectively, after a constant time lapse from the supply of ozone to the atmospheric pressure CVD chamber 8 to introduce TEOS gas into the atmospheric pressure CVD chamber 8 at a flow rate controlled by the mass flow controller 11, resulting in a thin SiO.sub.2 formed on a surface of the semiconductor wafer. After the completion of the formation of thin film, the states of the gas lines are returned to those prior to the film formation.
The mass flow controller and the mass flow meter will be described. FIG. 2 is a cross section of one (7) of the mass flow controllers 4, 7 and 11 which has the same construction. In FIG. 2, a gas flow passage includes an inlet pipe 51 which is branched to a bypass pipe 52 and a sensor pipe 53 which are joined to form an outlet pipe 54, a pair of heater coils 55 provided on an upstream portion and a downstream portion of the sensor pipe 53 having an inner diameter of about 0.3 mm, respectively, for heating the sensor pipe 53, a control portion 56 including a circuit which constitutes a bridge circuit together with the heater coils 55 and a valve 58. When gas flows through the sensor pipe 53, the equilibrium of the bridge circuit is broken by an amount of heat in the sensor pipe 53 and an output signal corresponding to the flow rate of the gas is output from the bridge circuit. The output signal is compared by the control portion 56 with an external flow rate setting signal 57 and a difference therebetween is output as a valve control signal 59 to the valve 58 to control an opening thereof to thereby control the flow rate of the gas.
The mass flow meter has the same construction as that of the mass flow controller except that the mass flow meter has no flow rate control valve such as the valve 58 and operates to merely display the flow rate.
In the prior art mass flow controller having the above mentioned structure, the temperature of a portion of the piping on the upstream side of the sensor pipe 53 on which the heater coils 55 are arranged is always at substantially room temperature during the gas flows through the mass flow controller.
In performing the atmospheric pressure CVD by generating ozone using oxygen gas added with a small amount of nitrogen gas, there is a problem that the conventional mass flow controller having the above mentioned construction and using an ozone gas line containing nitrogen gas can not exactly control the gas flow rate when an accumulated amount of flow rate of ozone gas is increased.
The present inventors had studied this problem and have found that a small amount of nitrogen pentoxide (N.sub.2 O.sub.5) which is one of nitrogen oxides is produced together with ozone generated in the ozone generator due to nitrogen gas added to oxygen gas for the purpose of restriction of the reduction of concentration of ozone with time and the problem is caused by deposition and accumulation of nitrogen pentoxide in the vicinity of the inlet portion of the sensor pipe of the mass flow controller, whose inner diameter is small and temperature is substantially room temperature.